Project Summary New drug delivery platforms are vitally needed for the targeted delivery of high-specificity therapeutics to sites of disease in order to maximize therapeutic efficacy while limiting off-target side effects. The majority of efforts currently underway for the development of such targeted drug delivery systems are focused on the development of synthetic nanoparticles, materials which are costly to produce, store, and distribute. Here, we propose to develop cost-effective, self-replicating and flexible, programmable designer probiotics for the targeted delivery of therapeutics directly to sites of disease. We propose to utilize a synthetic biology approach to genetically engineer a widely and safely administered probiotic, Escherichia coli Nissle 1917, to express a nanomachine that can secrete therapeutic payloads into the intestinal milieu. These designer bacteria will be equipped with a type 3 secretion system modified to secrete proteins into the intestinal lumen rather than their innate target, the cytosol of mammalian cells. As proof of concept and towards the development of these designer probiotics as therapeutics, we will engineer these designer probiotics to secrete a new class of well- documented therapeutic biomolecules of exquisite specificity, single domain antibodies, also referred to as VHH. We will focus on the delivery of VHH multimers that exhibit profound neutralizing activity, VHH-based neutralizing agents (VNAs), which inhibit the activity of essential bacterial toxins or proinflammatory cytokines. Furthermore, we will investigate the potential of these strains as novel therapeutic paradigms for the treatment of both intestinal infections and inflammation disorders. Specifically, we will investigate the efficacy of these strains in the prevention of treatment of Clostridium difficile infections (CDI), hemolytic uremic syndrome (HUS) and inflammatory bowel disease (IBD). We appreciate that there might be some concern regarding the administration of genetically modified bacteria as therapeutics. However, as we enter the `era of the microbiome,' it seems extremely likely that such interventions are to become an integral component of the armamentarium utilized to treat infections and inflammatory disorders, particularly those rooted in the gastrointestinal tract, especially because of the high likelihood that such agents can overcome many issues associated with the wide-spread usages of antibiotics and systemic immunosuppressive agents. While efforts here are specifically devoted towards the development of these designer probiotics for the treatment of CDI, HUS, and IBD, once established as a therapeutic paradigm, this designer probiotic platform can be extended to treat a variety of intestinal based diseases. For example, the designer probiotics could be programmed to deliver a variety of protein-based therapeutic payloads, including cytokines, such as IL-10, that suppress intestinal inflammation or VNAs designed to target essential exposed virulence proteins of enteric bacterial pathogens, e.g., adhesins or essential components of virulence factor delivery systems.